Development of the ovarian follicle is a complex process that involves interplay between the multiple cell types of the follicle and requires the actions of both locally produced and extragonadal factors. Understanding the control of follicle development has important medical implications for the control of fertility and the treatment of infertility. We have generated two novel transgenic mouse models to investigate the role of the TGFbeta family protein activin in normal and aberrant development and ovulation of the ovarian follicle. Our preliminary studies reveal a remarkable spectrum of common ovarian pathologies in mice that express transgenes for either the alpha subunit of the activin antagonist inhibin (MT-alpha) or a dominant-negative Smad2 transgene (Smad2-DN). In each case, the female mice are subfertile and both abnormal multi-oocytic follicles and large cysts are found in the ovary. The common element linking these observations is a specific deficiency in activin production or action. In the inhibin alpha subunit mice, the excess alpha subunit forms inhibin heterodimers with endogenous activin beta subunits, thus locally reducing activin levels. In the Smad2-DN mice, Smad2, a key element of the activin signaling pathway, is inhibited. Thus, the central hypothesis of this proposal is that activin plays a critical role in normal development of the ovarian follicle. The studies proposed will test this hypothesis by investigating three related specific aims. Aim 1 will examine the development ofmulti-oocytic follicles and explore roles for activin in cell-cell interactions leading to the establishment of an appropriate follicle boundary. Aim 2 will determine whether neonatal estrogen exposure, a known inducer of multi-oocytic follicles, acts to regulate activin or activin signaling proteins, thus establishing a mechanistic linkage for the formation of multi-oocytic follicles in these two models. Aim 3 will investigate the formation of ovarian cysts in these transgenic mouse models and establish their origin, association with the ovulatory process and the ovarian surface epithelium, and regulation by activin. In total, the studies described in this application will provide new insights into normal follicle development, establish the mechanisms by which normal development is disrupted leading to aberrant ovarian pathologies such as multi-oocytic follicles and cysts, and offer new insights into the functions of activin regulation in each of these important reproductive processes.